This invention pertains generally to microwave signal sources and more particularly to a frequency-agile microwave signal source that is phase-locked to a selected one of the harmonics of a reference signal source.
As is known in the art, microwave signal sources are necessary elements in the transmitter and the receiver sections of a radar system. In many applications, it is highly desirable, if not essential, that such sources be frequency-agile, meaning that their frequencies may be changed during operation. One known way of providing such agility is to phase-lock a voltage-controlled oscillator (VCO) operating in the desired microwave band to a selected one of a number of harmonics of a reference signal source operating at a relatively low frequency. It will be evident that, to achieve frequency agility, there must be at least two harmonics (in practice there are far more) of the reference signal generator in the desired microwave band and that means must be provided to select the proper harmonic at any given time.
A conventional approach is to use a crystal-controlled oscillator operating, say, in the range of 40 megahertz, with appropriate frequency multipliers to produce harmonics in the desired microwave range, say 5 gigahertz. The VCO then is phase-locked to a selected one of the harmonics, the selected one being changed as required to achieve frequency agility. The circuitry for controlling the selection of any particular harmonic is, however, rather complicated with stringent limits on accuracy so it is difficult in practice to implement.